x^3+3x^2-4
=(x³－x²)＋(4x²－4)
=x²(x－1)＋4(x²－1)
=x²(x－1)＋4(x＋1)(x－1)
=(x－1)(x²＋4x＋4)
=(x－1)(x＋2)²